The broad, lone term objective of the proposed research is to understand cell cycle regulation. This may lead to new strategies for the control of health problems associated with the cell cycle, such as cancer and growth defects. p57 can arrest the cell cycle by inhibiting the catalytic activity of cyclin-dependent kinases (Cdks). p57 thus acts as a potential negative growth regulator and candidate tumor suppressor. The proposed research will use modern biophysical and molecular biology techniques to elucidate the mechanism of cyclin A-Cdk2 inhibition by p57. The specific aims are: 1. Determine the composition of the p57 inhibited cyclin A-Cdk2 complex using sedimentation equilibrium. 2. Characterize the structure of the p57 Cdk-inhibition domain by nuclear magnetic resonance. 3. Identify the amino acid residues of p57 that contribute to cyclin A-Cdk2 inhibition by site-directed mutagenesis of p 57. 4. Identify the amino acid residues of cyclin A and/or Cdk2 that interact with p57 by site-directed mutagenesis of cyclin A and Cdk2.